1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for forming a field oxide film of a highly integrated semiconductor device.
2. Description of the Prior Art
Generally, semiconductor devices have a field oxide film which is formed using an oxidation of silicon. Such a field oxide film is adapted to isolate adjacent active elements.
In the formation of a field oxide film, various conditions are required. For example, the field oxide film should have a short bird's beak in order to provide a wide active region. The field oxide film should also have a sufficient thickness even in a narrow oxidation window.
Where such a field oxide film has a certain large thickness, an increase in the threshold voltage of an associated parasitic transistor can be exhibited. In this case, an increase in punchthrough voltage is also exhibited.
Where a field oxide film is formed using the silicon oxidation, it is sufficiently grown in a wide field region to have a sufficient thickness while it is insufficiently grown in a narrow field region.
For instance, where a silicon oxidation is carried out in such a manner that a field oxide film is grown to a thickness of 3,000 .ANG., there is a phenomenon that the field oxide film is grown only to a thickness of 2,000 .ANG..
Such a phenomenon is called a "field oxide thinning effect". Where a design rule of 0.5 .mu.m or less is used, it is impossible to form a field oxide film having a sufficient thickness.
Where a field oxide film is grown in such a manner that it has a very large thickness in a wide field region, it can have a more or less sufficient thickness in a narrow field region. In this case, however, there is a problem in that an elongated bird's beak is formed.
In the formation of a field oxide film, consequently, it is required to sufficiently grow the field oxide film in a narrow field region while preventing the field oxide film from having a large thickness in a wide field region.
Although several theories are known in association with the cause of the field oxide thinning effect, the commonly recognized theory is a stress theory. J. W. Lutze, et al. demonstrated that a field oxide thinning phenomenon occurring in a poly buffer local oxidation silicon (LOCOS) (PBL) structure is caused by internal compressive stress existing in the filed oxide film ("Filed oxide thinning in poly buffer LOCOS isolation with active area spacings to 0.10 .mu.m", Journal of Electrochemical Society, Vol. 137, No. 6, p1867-1870 (1990)).
That is, as the oxidation proceeds, the field oxide film expands in volume while compressive stress existing in the field oxide film in a narrow field region increases relatively. As a result, the oxidation rate decreases.
Meanwhile, current semiconductor devices use a design rule of sub-quarter microns or less not allowing the formation of the least bird's beak.
For such semiconductor devices, it is impossible to use a structure, in which its pad oxide film is exposed, fabricated in accordance with the LOCOS or PBL technique. This is because such a structure can not prevent the formation of bird's beak.
In this regard, a technique, which fabricates the structure of FIG. 1, has been highlighted. The structure of FIG. 1 has a semiconductor substrate 1 formed with a recess having a certain depth in order to increase the volume ratio of a field oxide film formed over the semiconductor substrate 1, namely, the ratio of the thickness of a portion of the field oxide film disposed in the recess to the thickness of the other field oxide film portion. This structure also has a pad oxide film 2 formed over the silicon substrate 1 and spacers comprised of an oxidation-resistant film such as a nitride film and adapted to shield exposed portions of the pad oxide film 2.
Even in this structure, however, the field oxide thinning phenomenon, which occurs in LOCOS or PBL structures, is inevitably generated because the field oxide film is grown in accordance with an oxidation. Furthermore, this structure, which uses nitride film spacers as mentioned above, also involves the generation of stress having an influence on the field oxide thinning phenomenon, in addition to the generation of internal stress in the field oxide film demonstrated by Lutze, et al. This is because, the silicon substrate portions 5 disposed beneath the nitride film spacers 4 are subjected to compressive stress.
The nitride of the nitride film spacers 4 is a material having an intrinsic tensile stress. For this reason, the silicon substrate portions 5 is applied with compressive stress from the nitride film. It is difficult for oxygen to penetrate the silicon of the silicon substrate portions 5 where compressive stress is accumulated. As a result, an increased field oxide thinning effect is exhibited.
FIG. 2 is a diagram depicting a stress distribution in the structure of FIG. 1, which is measured by a Tsuprem-4 simulation after the formation of the nitride film spacers 4.
Referring to FIG. 2, it is understood that a concentration in compressive stress occurs at the silicon substrate portions respectively disposed beneath the nitride film spacers 4.
Accordingly, it is possible to reduce or eliminate the field oxide thinning effect by relieving the concentrated compressive stress because the field oxide thinning effect is caused by the compressive stress.
Two parameters associated with the relief of such compressive stress are known.
One parameter is "temperature". For instance, an easy relief of compressive stress is achieved at a high temperature because the viscosity of solids decreases at the high temperature so that the flowability of the solids increases.
Accordingly, a high temperature field oxidation has been used in the conventional PBL method in order to relieve internal stress existing in field oxide films.
In the case of the structure shown in FIG. 1, it is also possible to relieve the stress of the silicon portions 5 disposed beneath the nitride film spacers 4 as well as the internal stress of the field oxide film by carrying out the field oxidation at a high temperature. Accordingly, a reduction or elimination of the field oxide thinning effect is achieved.
FIG. 3 is a graph depicting the influence of oxidation temperature on the field oxide thinning effect in accordance with the conventional technique.
In FIG. 3, the y axis represents a variation in the thickness of a field oxide film grown in a narrow oxidation window while varying the field oxidation temperature under the condition in which the field oxide film has a constant thickness of 3,000 .ANG. in a wide field region.
The thickness variation is measured in percentage. For example, "50%" means that the field oxide film has a thickness of 1,500 .ANG. in the narrow oxidation window. Referring to the graph, it is understood that the field oxide film has an increased thickness in the narrow field region at an increased field oxidation temperature.
The other parameter associated with the relief of stress is a "relief time".
At the same oxidation temperature, an increase in the relief time results in an increase in the amount of relieved stress.